1. Field of the Invention
The invention relates to a process for making microporous membranes that incorporate selected gas selective sites. Examples of selected gas selective sites include selective transition element complexes (TEC""s) for oxygen, amines and/or exposed cations for carbon dioxide, and copper(I) and/or silver(I) sites for carbon monoxide. The selected gas selective sites are incorporated onto internal surfaces of porous membranes in a controlled spacing and orientation manner and such resulting microporous membranes being suitable for separating and purifying selected gas from the selected gas containing gas mixtures. The invention also relates to the microporous selective membranes so made.
2. Description of the Prior Art
For many years, air has been separated and purified by cryogenic distillation, for which operating temperatures are set by the vapor-liquid equilibria of the liquefied mixture. Cryogenic separation is capital intensive, particularly for production rates below several hundred tons per day. Alternative technologies compete in the marketplace particularly for air separations to produce oxygen and nitrogen at lower purities and lower production rates than cryogenic systems. Pressure swing adsorption (PSA) has been applied to air separation and purification at near ambient temperature using either gas-solid equilibria or differences in uptake and release rates between adsorbates for a given adsorbent. Gas separation using membrane systems takes advantage of differences in permeation rates for the different feed components. Process aspects of membrane gas separation are simpler than either cryogenic or PSA systems.
Conventional air separation using membrane systems employs thin polymeric coatings which exhibit permselectivity for one or more components of the feed. For air separation, the primary concern is the separation of oxygen and nitrogen with the selectivity for other feed components such as water and carbon dioxide playing a lesser role. Advances in membrane materials have relied on the identification and utilization of polymers showing increased selectivity and permeability, combined with improvements in coating technology to give thinner separation layers.
Certain transition element complexes possess sites which show high selectivities for oxygen over nitrogen, argon, and other air components. Several attempts have been made to incorporate oxygen selective TEC sites into membrane systems to give rise to facilitated oxygen transport. Alternative approaches have been utilized such as those which are supported liquid membranes where the TEC sites function as mobile carriers or dense polymer membranes or microporous membranes where the TEC""s serve as fixed sites. For example, a membrane containing Co(3-MeOsaltmen) in xcex3-butyrolactone containing 4-(N,N-dimethylamino)pyridine (DMAP) at xe2x88x9210xc2x0 C. showed a selectivity xcex1(O2/N2)  greater than 20 with an oxygen permeability of 260 Barrer. Several problems have been identified which restrict or limit the application of supported liquid membranes to air separation. These include practical restrictions such as solubility limits for the TEC, membrane thickness, and low carrier mobility. In addition, decline in membrane performance can occur by irreversible TEC oxidation, evaporation loss of the liquid medium, and contamination of the liquid membrane with minor atmospheric components.
Permeation within polymeric materials can be described by a combination of diffusional and solubility effects. The incorporation of TEC""s into polymeric substrates has been disclosed to improve oxygen transport through solubility enhancement. Several alternative configurations have been examined such as examples where the TEC/axial base is physically incorporated in the polymer, or where the TEC and/or axial donor are covalently linked to the polymer.
Permeation studies for dense polymer membranes containing TEC""s have indicated that the increasing oxygen permeability with decreasing upstream oxygen pressure is consistent with facilitation. For example, permeation studies at 35xc2x0 C. for polybutylmethacrylate containing 4.5 wt. % 1-Melm/Co(TpivPP), where 1-Melm refers to 1-methylimidazole and Co(TpivPP) refers to meso-tetra (xcex1,xcex1,xcex1,xcex1-o-privalamidophenyl) porphyrinato-cobalt(II), indicated a P(O2) of 23 Barrer with selectivity xcex1(O2/N2) of 12 at 5 mmHg upstream pressure and lifetimes on the order of months.
Oxygen facilitation in styrene-butadiene-vinylpyridine graft copolymers, and epoxidized styrene-butadiene block copolymers containing TEC""s have been disclosed in the prior art. Pressure dependent oxygen permeation with xcex1(O2/N2) up to 6.2 and P(O2) 29.4 Barrer with an upstream pressure of 50 mmHg have been reported. Permeation studies for polyhexylmethacrylate-co-vinylpyridine containing N,N1-bis(salicylidene)ethylenediaminocobalt(II) (abbreviated as Co(salen)) prepared by an interfacial reaction between a polymer solution and a TEC impregnated in a porous membrane have been reported. Selectivities over 20 were reported with an oxygen permeation rate of 3.12xc3x9710xe2x88x928 cm3/cm2/s cmHg at 100 torr upstream pressure. In an alternative approach, a rigid porous support for butylacrylate-co-vinylimidazole membranes containing Co(TpivPP) have provided permeation measurements indicating xcex1(O2/N2) of 16.
Gas separation membranes consisting of TEC""s related to Co(salen) and DMAP physically incorporated in polysulfone which exhibit facilitated oxygen transport have been disclosed. A dry/wet phase inversion process was utilized to give thin separation layers and to prevent TEC crystallization. The stability of the separation membranes have been found to be satisfactory over 3 months using either a synthetic air or a compressed air feed. The highest selectivity reported corresponds to asymmetric membranes containing 15.1 wt. % Co(5-NO2-salen) which gives xcex1(O2/N2) of 23.46.
Carbon molecular sieve (CMS) membranes have been prepared by controlled carbonization of polymeric substrates, and permit gas separations on the basis of molecular dimensions. For the separation of components with similar kinetic diameters such as oxygen and nitrogen, fine control of the pore size distribution of the CMS membranes is generally required.
There are several examples of microporous membranes where surface diffusion plays a significant role in transport including the transport of organic vapor mixtures through porous glass, and gas permeation for porous glass modified using tetraethoxysilane. The transport of pure gases and binary gas mixtures through microporous composite membranes based on alumina have shown surface diffusion of carbon dioxide with a surface diffusion coefficient estimated to lie in the range 2xc3x9710xe2x88x925 to 5xc3x9710xe2x88x925 cm2/sec.
U.S. Pat. No. 5,104,425 describes porous membranes incorporating a adsorbent which separate gas mixtures by virtue of the selective adsorption of at least one primary component. The adsorbed component diffuses by surface flow in the adsorbed phase due to concentration gradients created by a pressure difference across the membrane. Pore diameters, for the substrate described in the patent should be between 0.1 and 50 xcexcm, and the thickness of the active layer containing porous adsorptive material should be  less than 201 xcexcm. The incorporation of porous adsorbent coatings involves introduction of an adsorbent precursor, heating to convert the precursor to the porous adsorbent, and cooling. Methods for the modification of the porosity of the adsorptive layer are disclosed, together with post-treatments for the sealing of defects using a thin layer ( less than 1 xcexcm) of high permeability, low selectivity polymer such as a silicon containing polymer. Permeation measurements, for example of an adsorbent membrane at 25xc2x0 C. indicate a(CO2/H2) of 7.1 and a CO2 permeability of 3360 Barrer.
Oxygen separating porous membranes have been disclosed in the prior art where oxygen-selective TEC""s are retained in the pores of a porous substrate to give membranes with free pore diameters from 3.5 to 100 xc3x85. An additional donor required to provide five-coordinate deoxy TEC sites is provided by aromatic heterocycles either from copolymers of vinyl aromatic heterocycles and alkyl-acrylates or -methacrylates, or from low molecular weight aromatic heterocycles. High oxygen selectivities were reported due to rapid adsorption and desorption of oxygen at TEC sites resulting in oxygen surface flow. For example, mixed gas permeation measurements using 2.6% oxygen in nitrogen as feed for a membrane consisting of Co(TpivPP) and poly(N-vinylimidazole-co-octyl methacrylate) (POMIm) supported in Vycor 7930 porous glass, indicate an xcex1(O2/N2) 7, with P(O2) 41,000 Barrer.
Although supported liquid membranes containing TEC""s as mobile carriers have shown promising permeation characteristics on a laboratory scale, they are not useful for practical gas separation. Several concerns have been identified for these systems. These include practical restrictions such as solubility limits for the TEC, membrane thickness, and low carrier mobility. In addition, decline in membrane performance has been observed due to irreversible TEC oxidation, evaporation loss of the liquid medium, and contamination of the liquid membrane with minor atmospheric components. The microporous oxygen selective membranes described in the subject invention are solid systems which avoid solubility, mobility, and stability concerns.
The application of dense polymer membranes containing TEC""s for gas separation are disclosed in the prior art. As in the case of simple polymer membranes, the membranes can incorporate TEC""s which separate gases on the basis of differential solubility and diffuisivity of gases within the material. Although promising permeation characteristics (oxygen selectivity and permeability) has been reported, membranes containing TEC""s have not been applied to commercial separations due to a number of factors. The ability to produce thin polymer films containing TEC""s which are mechanically stable and which contain low defects has not been demonstrated. For example, high TEC contents have been found to result in more brittle materials. In addition, current methods used to produce polymer membranes containing TEC""s for gas separation do not adequately control TEC spacing to prevent unfavorable processes including bimolecular decomposition pathways which are known to occur in liquids.
Liquid membranes containing TEC""s have shown interesting properties but have not proven useful for air separation due to irreversible TEC oxidation, evaporation loss of the liquid medium, and contamination of the liquid membrane with minor atmospheric components.
U.S. Pat. No. 5,411,580 discloses an oxygen-separating porous membrane comprising a complex of (a) a transition metal (II) ion, (b) a ligand, and (c) an aromatic heterocycle, retained in the pores of a substrate.
U.S. Pat. No. 5,229,465 discloses an oxygen-permeable polymeric membrane, intended for use in processes for producing oxygen- or nitrogen-enriched air for industrial, medical, and other applications, are characterized by a complex which comprises (a) a copolymer of a vinyl aromatic heterocycle and either (i) a fluoroalkyl acrylate or (ii) a fluoroalkyl methacrylate, and (b) a ligand taken from the group consisting of (1) porphyrins, (2) Schiff bases, (3) cyclidenes, and (4) amine-like macrocycles, and (c) a transition metal (II) ion.
U.S. Pat. No. 5,945,070 discloses a process for air separation using oxygen-selective sorbents with enhanced selectivity, loading capacities and oxygen uptake rates have a transition element complex in solid form supported on a high surface substrate. The transition element complex is substantially uniformly spaced, and includes a transition element ion accessible to an oxygen-containing gas stream during use in the separation of oxygen from an oxygen-containing gas mixture such as air.
It will be appreciated, therefore, that further improvements in the art are needed to enable membrane processes to satisfy the requirements of the art. In particular, further improvements are desirable with respect to transition element complexes in order to enhance the use thereof as oxygen-selective sites, especially solid TEC""s in supported form in which the spacing and orientation of the TEC sites are controlled. Similarly, selected gas selective sites can be chosen for separation of selected gas from its mixtures.
It is an object of the present invention to provide a process for producing a microporous membrane system which combine high permeabilities and selectivities for selected gases through the provision of selected gas selective sites for active surface diffusion.
It is another object of the present invention to provide a process for producing microporous selected gas selective membranes that incorporate selective sites for selected gases in which the spacing and orientation of the sites are controlled.
It is another object of the present invention to provide a microporous oxygen selective membrane containing TEC sites which are oxygen selective at equilibrium.
The invention relates to a process for producing a selected gas-selective microporous membrane and the membrane so produced and such membrane having a surface diffusion of a selected gas using selective sites which are selective at equilibrium employing reversible chemisorption and wherein said microporous membrane has a pore size of less than about 25 xc3x85 and said selected gas selective sites have a footprint of less than about 200 (xc3x85)2 and preferably having selected gas surface diffusion coefficients greater than 1xc3x9710xe2x88x925 cm2/s and preferably a thickness of less than about 2000 xc3x85.